Electronic devices, such as electroacoustic transducers, e.g., loudspeakers and microphones to be used in mobile phones; hard disk drives of personal computers; engine control units of automobiles; and digital cameras, each have at least one opening for allowing a sound to pass therethrough, or at least one opening for adjusting the atmospheric pressure in the inside of each electronic device; thus, each such electronic device foams a semi-enclosed space. The opening is provided with a vent film for the purpose of preventing the entering of water droplets, dust, harmful gases (e.g., Nox, Sox, oil mist), and others in each electronic device. The vent film requires waterproofness and dust-proofness, and also functions to allow a sound to pass therethrough and to adjust the internal pressure (see Patent Document 1). In this connection, the vent function of the vent film is exhibited or used, not only after the completion of electronic devices or others, but also during the manufacturing process of electronic devices (e.g., when ventilation is carried out so as to allow the escape of a gas expanded in the step of reflow-soldering electronic components).
As a material for forming a vent film as described above, porous polytetrafluoroethylene has widely been used (see, e.g., Patent Document 2).